This study is based on the hypotheses that: 1. Some thyroid carcinomas are epigenetic in nature, and 2. the expression of the embryonal and adult genome of these tumor cells may be influenced by environmental factors (hormones and humoral factors). Certain medullary carcinomas exhibit embryonal genome by secreting ACTH-like substances. Both the embryonal genome, and adult genome controlling normal structure and function are sequentially manifested in some human papillary- follicular thyroid carcinomas by forming metastases composed of normal- appearing and functioning thyroid follicles. The same phenomenon was observed when trypsin-dispersed cells of some transplantable papillary- follicular thyroid tumors of the rat (TTTR) were injected intracardially in the same inbred species of rats. Therefore, the following studies are proposed. 1. Cell culture of cloned TTTR and human thyroid carcinomas to define the structure, function and growth of tumor cells displaying embryonal and adult genome. Follicle formation and electromicroscopic structure, metabolism of I131, cell cycle and its phases of these cells (H3-thymidine) will be evaluated in vitro and when suitable, in vivo. 2. Assay of carcino-embryonic antigen CEA in: a) plasma and extracts of medullary and papillary-follicular carcinoma, and b) extract of TTTR and plasma of the hosts. 3. Radioimmunoassay of serum thyroglobulin in human and rats with differentiated thyroid carcinoma. These studies could: a) evaluate CEA assay in diagnosis and classification of some thyroid carcinoma and b) further understanding the epigenetic mechanism of carcinogenesis and normal regulation of cell growth.